A system, such as an infrared system, may realize wavelength selectivity by, for example, filtering selected photons, or, for example, by selective detection of photons having a particular wavelength. Further, a system, such as an infrared system, may realize wavelength tunability by tuning a filter that filters selected ones of the photons. Such selectivity and/or tunability may, in part, provide electrically tunable infrared detectors. Electrically tunable infrared detectors are highly desirable for advanced sensing and imaging applications and systems.
Tunable photon filters are a simple and common method for the realization of tunable infrared focal plane arrays. Discrete and continuous tuning have been realized by placement of filter wheels, tunable Fabry-Perot filters, or FTIR based filters in front of the focal plane arrays. However, wavelength tunability at a “pixel level” cannot be achieved by these methods.
A pixel-level integration of tunable filter elements and detector elements is highly desirable. Such integration is difficult to develop for mass production, and has several physical limitations. Such difficulties are due, in part, to the absence of an effective multi-level heterogeneous integration method. More fundamentally, such difficulties are due, in part, to physical separation of the filter and detector, which makes the broadband detector more vulnerable to interference, such as pixel-to-pixel crosstalk, wavelength suppression, and the like. This vulnerability becomes more significant as electrical interconnections and/or electronic components are placed between the elements of the filter and the elements of the detector. Additionally, thermal, mechanical, and electrical incompatibilities between the material of the filter and the material of the detector may limit the operation of the device to undesirably narrow temperature, acceleration, and voltage ranges.
These issues may, in part, be eliminated or alleviated if filtering and sensing were unified. For example, selective detection integrates the filter and the sensor elements. An absorption mechanism for selective detection may be provided by resonance between photons and electron states, which provides an avenue for selective infrared detection because only photons with energies equal to the difference of the energy levels can excite electrons. The selectivity of this absorption mechanism is illustrated by the narrow absorption lines in gases with Δλ/λ<0.1%.
In semiconductor material, the interaction between periodically spaced atoms provides a continuous range of allowed energy states, and this continuous range is termed an “energy band.” Photon absorption between two electron energy bands in a semiconductor material is not highly selective. Commonly used devices based on inter-sub-band transition include quantum well infrared photodetectors (QWIP) with a natural bandwidth Δλ/λ of only 10–20%. Unfortunately, this bandwidth is not narrow enough for many applications, and it cannot be easily tuned.
A principal absorption broadening mechanism in a quantum well inter-sub-band transition is energy dispersion in the plane of quantum wells. Such energy-momentum dispersion is a direct result of atomic periodicity generating a periodic potential in the plane of the quantum wells.